Flavor pleasantness processing in the ventral emotion network

A mesocorticolimbic signature of pleasure in the human brain

Pleasure is a fundamental driver of human behaviour, yet its neural basis remains largely unknown. Rodent studies highlight opioidergic neural circuits connecting the nucleus accumbens, ventral pallidum, insula and orbitofrontal cortex as critical for the initiation and regulation of pleasure, and human neuroimaging studies exhibit some translational parity. However, whether activation in these regions conveys a generalizable representation of pleasure regulated by opioidergic mechanisms remains unclear. Here we use pattern recognition techniques to develop a human functional magnetic resonance imaging signature of mesocorticolimbic activity unique to states of pleasure. In independent validation tests, this signature is sensitive to pleasant tastes and affect evoked by humour. The signature is spatially co-extensive with mu-opioid receptor gene expression, and its response is attenuated by the opioid antagonist naloxone. These findings provide evidence for a basis of pleasure in humans that is distributed across brain systems.

Amplitude of low-frequency fluctuation after taste exposure revealed by resting-state fMRI

Taste perception has been deeply explored from the behavioural level to delineating neural mechanisms. However, most previous studies about the neural underpinnings of taste perception have focused on task-related brain activation. Notably, evidence indicates that task-induced brain activation often involves interference from irrelevant task materials and only accounts for a small fraction of the brain's energy consumption. Investigation of the resting-state spontaneous brain activity would bring us a comprehensive understanding of the neural mechanism of taste perception. Here we acquired resting-state functional magnetic resonance imaging (rs-fMRI) data from twenty-two participants immediately after they received sweet, sour and tasteless gustatory stimulation. Our results showed that, in contrast to the tasteless condition, the sour exposure induced decreased amplitude of low-frequency fluctuation (ALFF) in the somatosensory cortex in the left post-central gyrus, and the sweet exposure led to increased ALFF in the bilateral putamen involved in reward processing. Moreover, in contrast to the sweet stimulation condition, the sour stimulation condition showed increased ALFF in the right superior frontal gyrus, which has been linked to functioning in high-order cognitive control. Altogether, our data indicate that taste exposure may affect the spontaneous functional activity in brain regions, including the somatosensory areas, reward processing areas and high-order cognitive functioning areas. Our findings may contribute to a further understanding the neural network and mechanisms after taste exposure.

Functional Connectivity of the Chemosenses: A Review

Functional connectivity approaches have long been used in cognitive neuroscience to establish pathways of communication between and among brain regions. However, the use of these analyses to better understand how the brain processes chemosensory information remains nascent. In this review, we conduct a literature search of all functional connectivity papers of olfaction, gustation, and chemesthesis, with 103 articles discovered in total. These publications largely use approaches of seed-based functional connectivity and psychophysiological interactions, as well as effective connectivity approaches such as Granger Causality, Dynamic Causal Modeling, and Structural Equation Modeling. Regardless of modality, studies largely focus on elucidating neural correlates of stimulus qualities such as identity, pleasantness, and intensity, with task-based paradigms most frequently implemented. We call for further "model free" or data-driven approaches in predictive modeling to craft brain-behavior relationships that are free from a priori hypotheses and not solely based on potentially irreproducible literature. Moreover, we note a relative dearth of resting-state literature, which could be used to better understand chemosensory networks with less influence from motion artifacts induced via gustatory or olfactory paradigms. Finally, we note a lack of genomics data, which could clarify individual and heritable differences in chemosensory perception.

Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability

BACKGROUND

Insight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown.

METHODS

Bidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences.

RESULTS

Photoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning.

CONCLUSIONS

These results demonstrate a novel contribution of vHipp-NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp-NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people's food pleasantness ratings, hunger, and weight.

Intensity-related distribution of sweet and bitter taste fMRI responses in the insular cortex

The human gustatory cortex analyzes the chemosensory properties of tastants, particularly the quality, intensity, and affective valence, to determine whether a perceived substance should be ingested or rejected. Among previous studies, the spatial distribution of taste intensity-related activations within the human insula has been scarcely addressed. To spatially characterize a specialized or distributed nature of the cortical responses to taste intensities, a functional magnetic resonance imaging study was performed at 3 T in 44 healthy subjects where sweet and bitter tastants were administered at five increasing concentrations and cortex-based factorial and parametric analyses were performed. Two clusters in the right middle-posterior and left middle insula were found specialized for taste intensity processing, exhibiting a highly nonlinear profile across concentrations. Multiple clusters were found activated by sweet and bitter taste stimuli at most concentrations, in the anterior, middle-posterior, and inferior portion of the bilateral insula. Across these clusters, respectively, for the right and left insula, a superior-to-inferior and an anterior-to-posterior spatial gradient for high-to-low concentrations were observed for the most responsive intensity of both tastes. These findings may gather new insights regarding how the gustatory cortex is spatially organized during the perceptual processing of taste intensity for two basic tastants.

fMRIPrep: a robust preprocessing pipeline for functional MRI

Preprocessing of functional MRI (fMRI) involves numerous steps to clean and standardize data before statistical analysis. Generally, researchers create ad-hoc preprocessing workflows for each new dataset, building upon a large inventory of tools available. The complexity of these workflows has snowballed with rapid advances in acquisition and processing. We introduce fMRIPrep, an analysis-agnostic tool that addresses the challenge of robust and reproducible preprocessing for fMRI data. FMRIPrep automatically adapts a best-in-breed workflow to the idiosyncrasies of virtually any dataset, ensuring high-quality preprocessing with no manual intervention. By introducing visual assessment checkpoints into an iterative integration framework for software-testing, we show that fMRIPrep robustly produces high-quality results on a diverse fMRI data collection. Additionally, fMRIPrep introduces less uncontrolled spatial smoothness than commonly used preprocessing tools. FMRIPrep equips neuroscientists with a high-quality, robust, easy-to-use and transparent preprocessing workflow, which can help ensure the validity of inference and the interpretability of their results.

Activity of frontal pole cortex reflecting hedonic tone of food and drink: fNIRS study in humans

Cognitive and hedonic aspects of taste have been studied using different neuroimaging techniques in humans. However, the methods used are unsuitable for easy monitoring of hedonics induced by intake of foods and beverages. Here we have tried to monitor changes in oxygenated hemoglobin (oxyHb) levels in the anterior prefrontal cortex (aPFC, frontopolar cortex, Brodmann area 10) in response to intake of hedonically different edibles in healthy adults. When subjects tasted sweet and bitter solutions freely without any particular instruction, cortical activation varied greatly among subjects and between the two stimuli, and no consistent results were obtained. Subjects then ate or drank preferred (hedonically positive) and disliked (hedonically negative) edibles. Although these stimuli differed among subjects, hedonically positive stimuli decreased oxyHb, whereas hedonically negative stimuli increased oxyHb, particularly in the ventral aPFC. When subjects tasted 4 kinds of jellies with different flavors and evaluated the degree of pleasantness, oxyHb level in the ventral region correlated negatively with pleasantness score. These results revealed that pleasant and unpleasant edibles tended to elicit decreased and increased oxyHb levels, respectively, within the ventral aPFC, suggesting that monitoring of oxyHb in this region may prove useful for objective evaluation of pleasantness of food and drink.